Magnesium base alloys



United States Patent Claims priority, application Great Britain, Nov.15, 1963,

45,142/ 63 15 Claims. (Cl. 75-168) This invention relates to magnesiumbase alloys containing at least 80 percent magnesium. It is a commonexperience in magnesium base alloys that a grain boundary phase may havean embrittling or other deleterious effect on the alloy and the objectof the present invention is to reduce this harmful effect.

As an instance, the addition of rare earth metals to magnesium results,when the rare earth metal exceeds a certain percentage, in a brittlegrain boundary phase. Providing that the addition of rare earth metaldoes not exceed a known percentage, the grain boundary phase may betaken into solution by heat treatment in known manner and theembrittling effect is thus reduced. If the amount of rare earth metalsexceeds the limit of solid solubility, however, complete solution of thephase by heat treatment in known manner is impossible. Further, it isknown that if the magnesium base alloy also contains zinc, the grainboundary phase resulting from addition of rare earth metal is verystable and cannot be dissolved by heat treatment in known manner.

It is further known that the addition of certain elements such, forinstance, as rare earth metals and thorium to magnesium alloys,particularly those containing zinc, results in technical advantages,e.g., improved soundness in castings, and improved ability to work thematerial by plastic deformation. The use of additions of rare metals orthorium is presently limited by the embrittling effects which result.

According to one aspect of this invention a magnesium alloy is providedwith an alloying constituent consisting of at least one of the elementsrare earth metals and thorium, some at least of which is incorporated ina grain boundary phase, and the alloy is then heated in the presence ofhydrogen to effect reaction of hydrogen with said constituent and tocause part at least of the grain boundary phase to diffuse into the basemetal.

The term rare earth metals for the purpose of the I present inventionincludes yttrium. I

For the purpose of this invention the alloying element which is to reactwith hydrogen may be termed the active constituent.

The hydrogen may, for example, react with the active constituent in thegrain boundaries to form hydride thus attacking the grain boundary phaseand liberating one or more of its constituents to go into solution inthe mag nesium. Or, for further example, the hydrogen may react with theactive element already dissolved in the alpha phase, permitting solutionfrom the grain boundary phase with further progressive attack on thedissolved active constituent.

A particular application of the invention is in magnesium base alloyscontaining zinc. For example, a magnesium base alloy having high roomtemperature strength contains zinc 6% and zirconium 0.6 to 0.9%. Thisalloy, however, has a marked tendency to exhibit microporosity in thecast form, such porosity resulting in considerable reduction ofstrength. This tendency to exhibit microporosity has restrictedcommercial exploitation of the alloy. It is further well known that thetendency to microporosity of this alloy may be improved by addition ofrare earth metals, e.g., such as cerium mischmetal or 3,334,998 PatentedAug. 8, 1967 didymium mischmetal which are two forms of commerciallyavailable rare earth metal. Such additions, however, reduce the tensilestrength of the alloy and render it unattractive commercially. Inaccordance with one example of the present invention, however, one ormore rare earth metals may be included in the alloy and the alloy isthen heat treated in the presence of hydrogen so that the hydrogenreacts with one or more of the rare earth metals. The results given inthe following table illustrate the improvement in properties which canbe obtained by using the novel procedure described in this application:

MAGNESIUM-BASE ALLOY CONTAINING NOMINALLY 6% ZINC AND 0.7% ZIRCONIUM0.1% Proof Tensile Properties Heat Treatment Atmosphere Stress 0.2%U.T.S. E.,

'ISl P.S. TSl percent 'ISl S0, (Normal atmosphere for mag. based alloys)11. l 12. l 17. 5 3 Hydrogen 10. 7 11.7 16. 4 2

MAGNESIUM-BASE ALLOY CONTAINING NOMINALLY 6% ZINC, 2% R E METALS, 0.7%ZIRGQNIUM SO; 5. 3 6. 0 ll. 3 4 Hydrogen 10, 4 11. 7 19. 2 10. 5

The above results were obtained from cast test bars made in accordancewith British Standards Specification L101, FIG. No. 1. The heattreatment in each case was for 24 hours at 500 C. in S0 or hydrogenatmosphere followed by 64 hours at 125 C. in air.

In the case of the alloy detailed in the above example the improvementin mechanical properties resulting from application of this novelprinciple is associated with the modification of a metallurgical phasepresent in the grain boundaries of the alloy. The presence of this phaseis both responsible for the improvement in respect of microporosity andalso the resultant loss in tensile properties. The phase is normallyvery stable and cannot readily be dissolved by heat treatment inpreviously known manner. It is believed that the introduction ofhydrogen during heat treatment converts the rare metal content tohydride, resulting in breakdown of the original phase and thus removingthe deleterious eifect on strength. The probability of an improvement instrength when using any given active constituent can, therefore, beassessed by comparative metallographic examination of specimens heattreated in previously known manner with those heat treated according tothe present application.

By such comparative metallographic examination it has been found thatsuitable phase changes occur in magnesium alloys containing thorium sothat rare earth metals may be partly or wholly replaced by thorium.Yttrium may also be used as an active constituent.

The present invention, therefore, includes a magnesium base alloyconsisting apart from impurities of:

RE percent by Weight 0.2-6 Zinc d0 0.25-10 H cc./ g 1 At least 50Zirconium percent by weight 0-1 Manganese d0 0-2.5

Approx. 0.005% by weight.

In alloys containing both zirconium and manganese, if either is at least0.3% the other will not exceed 0.2%.

In alloys which do not contain zirconium and Where iron is included incontents of e.g. 0.03 to 0.1%, the silicon content will be less than0.05%

If desired, one or more of the following elements may be included in thealloy:

Percent by weight The following more limited range of composition ispreferred:

RE metals percent by weight 0.75-4 Zn do 3-8 Zr do 0.3-1.0 Ag do 0.25Hydrogen cc./ 100 g At least 50 For high ductility combined with asatisfactory proof stress the RE and zinc contents may be restricted tothe following ranges:

Percent by weight RE 0.75-1.5 Zn 3.5-5

The following RE and zinc ranges provide high proof stress combined withgood ductility:

Percent by weight RE l.75-4.0

For the highest proof stress values the following composition ispreferred:

Percent by weight RE 1.75-4 Zn 5-8 Zr 0.3-1.0 Ag 0.5-5

Since the proportion of certain of these elements (par ticularly zinc)is in practice limited by their unfavourable effect on suchcharacteristics as porosity, tendency to crack during solidification,weldability, etc., and since the addition of the constituents listed asactive in this application alleviates such tendency, the use of theprinciples disclosed in this application make it possible to increasethe permissible amounts of certain of these alloying constituents beyondthe levels found in present practice and thus make available alloys ofhigher strength.

Since addition of the active constituents is known to suppress thetendency of a magnesium-base alloy to crack when cooled rapidly (e.g. byquenching in water or oil) from a high temperature, it is expected thatthe use of the principle herein disclosed will permit such rapid coolingin alloy systems in which it is not now practicable. Such rapid coolingwould be expected to still further im prove the strength of the alloy.

In respect of silver as an alloying element it is known that theaddition of silver, e.g. up to 6%, to the magnesium base alloycontaining 6% zinc and 0.7% zirconium re sults in significantimprovement to tensile strength. Such addition, however, results in nobenefit to the marked tendency of this alloy to exhibit microporosity inthe cast form and hence the silver containing alloys are also restrictedin commercial exploitation by virtue of this characteristic. It has beenfound that the application of the principle herein disclosed alsoprovides for significant improvement in freedom from porosity of thesilver containing alloy with subsequent recovery of properties by theheat treatment in hydrogen. The addition of silver appears to reduce therate at which attack of the grain boundary phase by hydrogen occurs butcomplete or substantially complete conversion of the phase may beeffected by somewhat more prolonged treatment as compared with thesilver free alloy.

In respect of thorium used as an active alloying element it is knownthat the grain boundary phase formed, particularly when zinc is alsopresent, has a lower embrittling effect than in the case of rare earthmetals although for many applications the embrittling effect is toosevere. It may therefore be advantageous to use thorium wholly or partlyto replace rare earth metals as the active constituent particularlywhere only partial conversion of the grain boundary is desired. For thispurpose the thorium content may be 0.5-2.5 and the RE content 0.75-2.5%.If desired rare earth metals may be used in which the lanthanum contenthas been diminished, e.g. didymium.

It is well known that magnesium alloys for use at elevated temperaturesmay contain rare earth metals or thorium as an essential alloyingaddition. Alloys of this type when heat treated in the manner describedherein may suffer a loss of high temperature strength owing toconversion of the essential alloying addition to a form unsuitable forthe development of high temperature strength. The conversion of theactive element may be halted before completion, e.g. by suitable choiceof time and temperature, leaving the remainder of the active element toperform its other function of providing high temperature strength. Inthis manner alloys of improved castability and room temperature strengthbut still retaining high temperature strength may be produced.

A further advantage relates to alloys having high damping capacity. Itis known that a binary alloy of magnesium with nominally 0.6% zirconiumhas a high damping capacity, and this alloy is used commercially forthis purpose. This alloy, however, has very poor casting characteristicsand its commercial use for high damping purposes is limited by theinability to cast complex shapes. The addition of the activeconstituents herein listed, together preferably with an addition ofzinc, substantially improves the castability of this magnesium-zirconiumbinary alloy but that the addition of these active metals reduce thedamping capacity. Similarly pure magnesium and magnesium alloyscontaining manganese are known to have high damping capacity but are notused in practice owing to the inability to cast complex shapes. Thecastability of such alloys may also be improved by addition of theactive constituents herein listed but such additions in turn reduce thedamping capacity. The damping capacity of magnesium alloys containingactive elements may be improved by heat treatment in hydrogen. Preferredcompositions afiording good damping capacity combined with goodcastability are as follows:

RE Percent by weight 0.2-4 Zn do 0-3 Zr do 0.3-1.0 H cc./ g.-- At least50 When the allow does not contain zirconium, the RE content ispreferably 0.25-3%, with zinc 3.5-8% and manganese 0.15-2.5%, the silverbeing in the range 0- 0.25%, or 0.25-5% where improved properties aredesired. These compositions are also well suited to plastic working byknown processes.

The temperature of heat treatment of the magnesium alloy may be from C.to the immediate neighborhood of the solidus. Temperatures of at least450 C. will normally be used for the hydriding step and this willnormally be followed by a precipitation treatment at a temperature notexceeding 250 C.

In performing the heat treatment disclosed herein it is essential thatthe allow be permitted to absorb a significant quantity of hydrogen. Thehydrogen content of the magnesium alloy will be at least 50 ccs. per 100grams of the alloy and may be at least 80 ccs. The heat treatment may becarried out in an atmosphere of hydrogen or one rich in hydrogen, e.g.ammonia, hydrocarbon gases, etc., and also in an atmosphere containingmoisture such that the magnesium alloy reduces the moisture to liberatehydrogen. The heat treatment may also be carried out in any othermedium, e.g. a salt bath, providing that this is suited to the heattreatment of magnesiumbase alloys, e.g. nitrate baths are usuallyconsidered unsuitable owing to the risk of explosive reaction betweenmagnesimum and molten nitrates, but chloride baths could be used withoutsuch risk; and providing that hydrogen is made accessible to themagnesium alloy, e.g. by addition to the bath of hydroxides or unstablehydrides such as sodium hydride. The access of hydrogen in such a heattreatment may also be by an electrolytic process.

In carrying out the heat treatment it would be expected that the rate ofabsorption of hydrogen would increase if the pressure of thehydrogen-containing medium surrounding the magnesium was increased. Itwould further be expected that partial or complete ionisation of thehydrogen containing medium would increase the rate of hydrogenabsorption. This is in accordance with the known principles of gasabsorption. It has further been found that the magnesium alloy surface.may be beneficially treated to increase the rate of hydrogen absorption.Such treatments include shot blasting, providing a chromate film on themagnesium alloy surface, and applying salt solution to the magnesiumalloy surface.

It has further been found that the absorption of hydrogen may bedecreased or inhibited by other surface treatments. Magnesium surfacescleaned by anodising in a solution of ammonium bifluoride tend to resisthydrogen absorption; magnesium surfaces coated with fused boric acid maybe made to completely resist hydrogen penetration and similarly amagnesium alloy surface closely covered by a steel sheath has been shownto resist hydrogen penetration. It is therefore possible to producemagnesium alloy articles in which specific mechanical properties aredeveloped in selected parts of the article, the other parts havingspecific properties of a different nature. Thus by way of example a castarticle could be made in a magnesium-base alloy containing nominally 3%rare earth metals, 3% zinc and 0.6% zirconium. This alloy has goodstrength at elevated temperatures and has very good castability but hasa somewhat low ductility, of the order of 5% elongation in a tensiletest. By use of the invention the ductility can be increased to a valueof about 15% elongation, although in this specific alloy the newprinciple of treatment would be expected to result in some loss of hightemperature strength. By suitably masking selected areas of the castarticle, the hydriding treatment can be restricted to the unmaskedareas, providing a cast article with good creep resistance in selectedparts and good ductility in other selected parts.

It has further been found that by suitable choice of time, temperature,heat treatment medium, etc., the rate of progress of conversion of theactive element may be controlled to provide modified properties to arestricted depth below the surface of the article being treated. Theinvention can, therefore, be utilised to produce magnesium-base articleshaving strength characteristics significantly different at theirsurfaces compared with the characteristics towards the centre of theircross section.

It is further known that the addition of of the active metals listedherein improves the deformation characteristics of magnesium alloys buttheir use is limited by their effect in reducing strength. The alloysand methods disclosed herein may therefore also be applied to wroughtforms.

The invention enables castings to be produced possessing a minimum 0.1%proof stress of 9.0 t.s.i. combined with a minimum elongation value ofat least 7%, these figures relating to specimens cut from the casting.

I claim:

1. A magnesium base alloy comprising apart from impurities RE metalspercent by weight 0.2-6 Zinc do 0.25-10 Hydrogen ccs./ g. of alloy Atleast 50 Magnesium Balance 2. The alloy of claim 1 including a maximumof approximately 1 percent by weight of zirconium.

3. The alloy of claim 1 including a maximum of approximately 2.5 weightpercent manganese.

4. The alloy of claim 1 including a maximum of approximately 1 percentby weight zirconium and about 2.5 percent by weight manganese, thezirconium and manganese being so related that if either is at least 0.3percent the other will not exceed 0.2 percent.

5. A magnesium base alloy comprising the following composition:

RE metals percent by weight 0.25-3 Zinc do 3.5-8 Manganese do 0.15-2.5Hydrogen ccs./ 100 g. of alloy At least 50 Magnesium Balance 6. Thealloy of claim 5 including a maximum of about 0.25 percent by weightsilver.

7. An alloy as claimed in claim 1, together with one or more of thefollowing elements in the amounts stated:

Percent by weight 8. A magnesium .base alloy having the followingcomposition:

RE metals percent by weight-.. 0.75-4 Zinc do 3-8 Zirconium do 0.3-1.0Silver do 0*025 Hydrogen ccs./ 100 g. of alloy At least 50 MagnesiumBalance 9. A magnesium base alloy in accordance with claim 1 wherein theRE and zinc contents are as follows:

Percent by weight RE metals 0.75-1.5 Zinc 3.5-5

10. A magnesium base alloy in accordance with claim 1 wherein the RE andzinc contents are as follows:

Percent by weight RE metals l.754.0 Zinc 5.5-7.5

11. A magnesium base alloy having the following composition:

7 12. A magnesium base alloy having the following composition:

Percent by weight RE metals 0.752.5 Zinc 5-8 Zirconium 0.3-1.0 Silver0.5-5.0 Thorium 0.5-2.5 Hydrogen ccs./ 100 g. of alloy At least 50Magnesium Balance 13. A magnesium 'base alloy having the followingcomposition:

RE metals --percent by weight 0.25-3 Zinc do 3.5-8 Manganese do 0.15-2.5Silver do 0.25-5 Hydrogen ccs./100 g. of alloy At least 50 MagnesiumBalance 14. A sand casting in accordance with claim I possess-References Cited UNITED STATES PATENTS 3,101,269 8/1963 Emley 75l683,157,496 11/1964 Foerster 75l68 3,167,425 1/1965 Petch et al 75l683,183,083 5/1965 Foerster 75l68 DAVID L. RECK, Primary Examiner.

CHARLES N. LOVELL, Examiner.

11. A MAGNESIUM BASE ALLOY HAVING THE FOLLOWING COMPOSITION: